1. Field of the Invention
This invention relates to a novel epoxy resin curing system. The invention also is directed to cyanourea compounds in both monomeric and polymeric form which can be used as latent curing agents for epoxy resins. Additionally, the invention relates to formation of thermoset materials from epoxy resin-cyanourea compound formulations which can be used as one component adhesives sealants or coatings. This invention further relates to a process employing a novel curing agent in combination with an epoxy resin which, on application of heat, preferably in an accelerated manner, crosslinks to give a thermoset bond, seal or coating.
2. Description of the Prior Art
Conventional hot melt adhesive compositions are thermoplastic bonding materials which are solid at room temperature but become soft and fluid with good wettability of the adherent at elevated temperatures. These adhesives are readily applied in the molten state between adherends resulting in a strong adhesive thermoplastic bond on cooling and hardening.
Thermoplastic adhesives, which are used in the form of solutions, dispersions or solids, usually bond by purely physical means. Probably the most important means of applying thermoplastic adhesives is the hot melt method wherein bond formation occurs when the polymer melt solidifies in position between adherends. The bonds obtained by this method reach their final strength faster than those obtained from solution type adhesives. Obviously, the thermal stability of the thermoplastic resin determines its potential usefulness as a hot melt adhesive. In order for the thermoplastic to be used as a hot melt, it must also have a low melt viscosity, thus permitting application of the adhesive to the adherends at acceptable rates. Usually this means the polymer must have a low molecular weight. However, many thermoplastic materials cannot be employed as hot melts because they do not have sufficient cohesive strength at the low molecular weights required for application to a substrate. For example, the low molecular weight polyolefins, especially low molecular weight, low density polyethylene, are widely used in hot melt adhesives for sealing corrugated cartons, multy-wall bag seaming and the like, but they do not have sufficient strength to be used in structural applications such as plywood manufacture. Further, they do not have sufficient heat resistance to be used for bonding components which are intermittently exposed to elevated temperatures such as under the hood automotive applications. That is, thermoplastic adhesives cannot be employed where the adhesive in situ is reexposed to elevated temperatures which will cause the adhesive to sag thereby allowing the bond to break.
The concept of thermosetting or crosslinking resin adhesive is also known in the art. Many resin adhesives which undergo an irreversible, chemical and physical change and become substantially insoluble are known. Thermosetting adhesives comprising both condensation polymers and addition polymers are also known and examples include the urea-formaldehyde, phenol-formaldehyde and melamine-formaldehyde adhesives; epoxy, unsaturated polyester and polyurethane adhesives. More particularly, U.S. Pat. No. 3,723,568 teaches the use of polyepoxides and optional epoxy polymerization catalysts. U.S. Pat. No. 4,122,073 teaches thermosetting resin obtained from polyisocyanates, polyanhydrides and polyepoxides. Crosslinking in these patents is achieved by reaction with available sites in the base polymers. U.S. Pat. No. 4,137,364 teaches crosslinking of an ethylene/vinyl acetate/vinyl alcohol terpolymer using isophthaloyl, bis-caprolactam or vinyl triethoxy silane whereby crosslinking is achieved before heat activation with additional crosslinking induced by heat after application of the adhesive. U.S. Pat. No. 4,116,937 teaches a further method of thermal crosslinking by the use of polyamino bis-maleimide class of flexible polyimides, which compounds can be hot melt extruded up to 150.degree. C. and undergo crosslinking at elevated temperatures thereabove. In these latter two patents, thermal crosslinking is also achieved by reactions of the particular crosslinking agent with available sites of the base polymers. U.S. Pat. No. 3,934,056 teaches resin compositions of high adhesivity comprising ethylene-vinyl acetate copolymer, chlorinated or chlorosulfonated polyethylene, unsaturated carboxylic acids and an organic peroxide. Another thermosetting adhesive is known from U.S. Pat. No. 3,945,877 wherein the composition comprises a coal tar pitch, ethylene/vinyl acetate copolymer and ethylene/acrylic acid copolymer plus a crosslinking agent such as dicumyl peroxide.
In many of these prior art thermosetting adhesive compositions admixture of 2, 3 or 4 components is necessary in order to get a thermoset bond. Thus, the resultant bond depends on the homogeneity of the admixture. Further, in many cases, e.g., epoxy adhesives, two or more components must be admixed just prior to the preparation of the bond. This necessitates a fast application since the crosslinking reaction begins immediately upon admixture and is irreversible.
It is known to form 1-cyano-3-phenylurea. This product has a melting point in the range 122.degree.-126.degree. C. whereat it decomposes and fails to form a clear melt during the decomposition. See Organic Synthesis IV, pp. 113-217, J. Wiley & Sons, Inc., 1963. No use is known for this compound.
It is also known to cure epoxy resins with dicyandiamide. However, such a curing agent is foreign to those taught herein.